Solid free-form fabrication (SFF) is a designation for a group of processes that produce three dimensional shapes from additive formation steps. Most SFF processes are also referred to as layer additive manufacturing processes. SFF does not implement any part-specific tooling. Instead, a three dimensional component is often produced from a graphical representation devised using computer-aided modeling (CAM). This computer representation may be, for example, a layer-by-layer slicing of the component shape into consecutive two dimensional layers, which can then be fed to control equipment to fabricate the part. Alternatively, the manufacturing process may be user controlled instead of computer controlled. Generally speaking, a component may be manufactured using SFF by successively building feedstock layers representing successive cross-sectional component slices. Although there are numerous SFF systems that use different components and feedstock materials to build a component, SFF systems can be broadly described as having an automated platform/positioner for receiving and supporting the feedstock layers during the manufacturing process, a feedstock supplying apparatus that directs the feedstock material to a predetermined region to build the feedstock layers, and an energy source directed toward the predetermined region by a torch. The energy from the energy source modifies the feedstock in a layer-by-layer fashion in the predetermined region to thereby manufacture the component as the successive layers are built onto each other.
One recent implementation of SFF is generally referred to as ion fusion formation (IFF). With IFF, a torch such as a plasma, gas tungsten arc, plasma arc welding, or other torch with a variable orifice, in conjunction with a stock feeding mechanism to direct molten feedstock to a targeted surface such as a base substrate or an in-process structure of previously-deposited feedstock. A component is built using IFF by applying small amounts of molten material only where needed in a plurality of deposition steps. The result is a net-shape or near-net-shape part without the use of patterns, molds, or mandrels. The deposition steps are typically, but not always performed in a layer-by-layer fashion wherein slices are taken through a three dimensional electronic model by a computer program. Hence in most deposition applications it would be considered a layer additive manufacturing process. A positioner then directs the molten feedstock across each layer at a prescribed thickness.
There are also several other SFF process that may be used to manufacture a component. SFF processes can be sub divided into subcategories, two of which are direct metal deposition (DMD) and selective laser sintering (SLS). DMD is a process whereby metal is melted then placed where needed to build a three-dimensional part. SLS on the other hand spreads a layer of powder on a table then selectively fuses the appropriate portion to build a three-dimensional component. One of the challenges facing SFF processes, and more particularly ion fusion formation (IFF) processes and direct metal deposition (DMD) processes is that of achieving a sufficiently high deposition rate, so that the cost of the component being fabricated is reduced. In order to achieve higher deposition rates, high heat is required. This applies to all IFF and DMD systems but particularly to gaseous systems, such as arc based systems. These types of gaseous systems inherently tend to be more energy diffuse than laser or electron beam systems due to the basic mechanism of heat transfer, and more particularly the impingement of very high temperature gas flow onto a work piece. One inherent limitation of this type of system is the torch gas concentration mechanism, also referred to as the torch nozzle, and the velocity of the gas through the orifice of the torch nozzle.
A conflict exists between the need for high heat and accompanying high deposition rates and the life of the torch gas concentration mechanism, and more particularly the torch nozzle. In general, high heat is generated by an increase in gas flow. This increase in gas flow may be achieved by increasing the velocity or using a torch nozzle having a large orifice. An increase in the velocity of the gas moving through the orifice of the torch nozzle typically results in erosion of the nozzle. In addition, with an increase in the nozzle size, and more particularly the orifice diameter, the energy density is reduced and the deposition becomes coarser, and complicates the need for deposition accuracy. Accordingly, while one criterion for increased deposition rate can be achieved by flowing more gas through a larger nozzle, hence more heat, a larger nozzle creates a larger, less accurate deposit. The closer the solidified deposition is to final dimensions the less machining is needed and the lower the cost of the final fabricated product.
To deliver high heat with higher deposition accuracy, the orifice of the torch nozzle must be small, yet allow large amounts of heat to pass through. With a gaseous system to carry the increased heat, erosion of the nozzle orifice will occur. To prolong the life of the torch nozzle, the orifice must be kept cool and resistant to heat. Current DMD torch nozzles include copper as the most common nozzle material due to its ability to be kept cool. However, the copper/gas interface is susceptible to erosion due to the high heat. Copper has a low melting temperature compared to refractory metals and ceramics. Conversely, while refractory metals and ceramics are resistant to heat, most do not conduct heat as well as copper nor are they necessarily resistant to arc erosion.
In addition to SFF, joining of two components using conventional plasma torches and nozzles creates relatively large fusion zones compared to other fusion joining processes such as electron beam or laser welding. A narrower erosion resistant orifice could reduce the fusion zone width (diameter) of the plasma weld and possibly increase penetration of the weld. The latter would result from a higher energy density at the plasma spot in the joint.
Hence, there is a need for an erosion resistant torch for use in high heat applications, such as solid free-form applications, including a direct metal deposition system that includes a torch nozzle having an orifice that is resistant to high heat, thereby minimizing nozzle erosion and increasing the life of the torch.